Astro Mode Of Operation; Rf Basic Theory Of Operation; Figure 3. Rf Block Diagram For Vhf (Power And Control Omitted); Table 2.Local Oscillator And First If Frequencies - Motorola ASTRO XTSTM 2500 Basic Service Manual

Basic Theory of Operation
From TX_SSI
To RX_SSI
ASTRO Mode of
Operation
RF Basic
Theory of
Operation
8
Reference
Oscillator
Loop
EPIC
Filter
Y200
FracN
U201
3
MOD
IN
LPF
DAC
U203 FL200
ABACUS III U401
3
Sample 2ND
Clk LO

Figure 3. RF Block Diagram for VHF (Power and Control Omitted)

Transmitted signalling information is accepted by the DSP from the microcontrol
unit, coded appropriately, and passed to the D/A converter, which handles it the
same as a voice signal. Modulation information is passed to the synthesizer along
the modulation line. A modulated carrier is provided to the RF PA, which transmits
the signal under dynamic power control.
In the ASTRO mode (digital mode) of operation, the transmitted or received signal
is limited to a discrete set of deviation levels, instead of continuously varying. The
receiver handles an ASTRO-mode signal identically to an analog-mode signal up
to the point where the DSP decodes the received data. In the ASTRO receive
mode, the DSP uses a specifically defined algorithm to recover information.
In the ASTRO transmit mode, microphone audio is processed identically to an
analog mode with the exception of the algorithm the DSP uses to encode the
information. This algorithm will result in deviation levels that are limited to discrete
levels.
The receiver front end consists of a preselector, an RF amplifier, a second
preselector, and a mixer. Both preselectors are varactor-tuned, two-pole filters,
controlled by the microcontroller unit through the D/A IC. The RF amplifier is a dual-
gate, gallium-arsenide-based IC. The mixer is a double-balanced, active mixer,
coupled by transformers. Injection is provided by the VCO through an injection
filter. See Table 2 for local oscillator (LO) and first IF information.
Table 2. Local Oscillator and First IF Frequencies
LO Frequency Range
First IF Frequency
For the VHF band, the frequency generation function is performed by two ICs and
associated circuitry. The reference oscillator provides a frequency standard to the
TX Driver Power
Amplifier Module
TX
Q101
Buffer U101
Directional
Coupler
U106
VCO
RX
Buffer
RF Power
Detector
D103
VCO
PCIC
U102
Crystal Preselector RX LNA
Mixer
Filter Filter Q29
U1
FL51
VHF
--
44.85 MHz
Antenna
Switch
Harmonic
Filter
Attenuator
Preselector
Filter
MAEPF-27322-O
To
Antenna